Split-can dipole antenna for an implantable medical device

ABSTRACT

An apparatus and method for enabling far-field radio-frequency communications with an implantable medical device utilizing the device housing as an antenna. Such radio-frequency communications can take place over much greater distances than with inductively coupled antennas.

FIELD OF THE INVENTION

This invention pertains to implantable medical devices such as cardiacpacemakers and implantable cardioverter/defibrillators. In particular,the invention relates to an apparatus and method for enablingradio-frequency telemetry in such devices.

BACKGROUND

Implantable medical devices, including cardiac rhythm management devicessuch as pacemakers and implantable cardioverter/defibrillators,typically have the capability to communicate data with a device calledan external programmer via a radio-frequency telemetry link. A clinicianmay use such an external programmer to program the operating parametersof an implanted medical device. For example, the pacing mode and otheroperating characteristics of a pacemaker are typically modified afterimplantation in this manner. Modern implantable devices also include thecapability for bidirectional communication so that information can betransmitted to the programmer from the implanted device. Among the datawhich may typically be telemetered from an implantable device arevarious operating parameters and physiological data, the latter eithercollected in real-time or stored from previous monitoring operations.

Telemetry systems for implantable medical devices utilizeradio-frequency energy to enable bidirectional communication between theimplantable device and an external programmer. An exemplary telemetrysystem for an external programmer and a cardiac pacemaker is describedin U.S. Pat. No. 4,562,841, issued to Brockway et al. and assigned toCardiac Pacemakers, Inc., the disclosure of which is incorporated hereinby reference. A radio-frequency carrier is modulated with digitalinformation, typically by amplitude shift keying where the presence orabsence of pulses in the signal constitute binary symbols or bits. Theexternal programmer transmits and receives the radio signal with anantenna incorporated into a wand which can be positioned in proximity tothe implanted device. The implantable device also generates and receivesthe radio signal by means of an antenna, typically formed by a wire coilwrapped around the periphery of the inside of the device casing.

In previous telemetry systems, the implantable device and the externalprogrammer communicate by generating and sensing a modulatedelectromagnetic field in the near-field region with the antennas of therespective devices inductively coupled together. The wand must thereforebe in close proximity to the implantable device, typically within a fewinches, in order for communications to take place. This requirement isan inconvenience for a clinician and limits the situations in whichtelemetry can take place.

SUMMARY OF THE INVENTION

The present invention is an apparatus and method for enablingcommunications with an implantable medical device utilizing far-fieldelectromagnetic radiation. Using far-field radiation allowscommunications over much greater distances than with inductively coupledantennas. In accordance with the invention, separate conductive portionsof a housing for the implantable device act as a dipole antenna forradiating and receiving far-field radio-frequency radiation modulatedwith telemetry data. The antenna is dimensioned such that a substantialportion of the radio-frequency energy delivered to it at a specifiedfrequency by a transmitter in the implantable device is emitted asfar-field electromagnetic radiation. A tuning circuit may be used totune the antenna by optimizing its impedance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates one embodiment of a split-can dipole antenna.

FIG. 2 illustrates an alternate embodiment of a split-can dipole antennawith the device header separating the two housing portions.

FIG. 3 is a block diagram of the components of an exemplary cardiacrhythm management device.

DETAILED DESCRIPTION

As noted above, conventional radio-frequency (RF) telemetry systems usedfor implantable medical devices such as cardiac pacemakers utilizeinductive coupling between the antennas of the implantable device and anexternal programmer in order to transmit and receive RF signals. Becausethe induction field produced by a transmitting antenna falls off rapidlywith distance, such systems require close proximity between theimplantable device and a wand antenna of the external programmer inorder to work properly, usually on the order of a few inches. Thepresent invention, on the other hand, is an apparatus and method forenabling telemetry with an implantable medical device utilizingfar-field radiation. Communication using far-field radiation can takeplace over much greater distances which makes it more convenient to usean external programmer. Also, the increased communication range makespossible other applications of the telemetry system such as remotemonitoring of patients and communication with other types of externaldevices.

A time-varying electrical current flowing in an antenna produces acorresponding electromagnetic field configuration that propagatesthrough space in the form of electromagnetic waves. The total fieldconfiguration produced by an antenna can be decomposed into a far-fieldcomponent, where the magnitudes of the electric and magnetic fields varyinversely with distance from the antenna, and a near-field componentwith field magnitudes varying inversely with higher powers of thedistance. The field configuration in the immediate vicinity of theantenna is primarily due to the near-field component, also known as theinduction field, while the field configuration at greater distances isdue solely to the far-field component, also known as the radiationfield. The near-field is a reactive field in which energy is stored andretrieved but results in no net energy outflow from the antenna unless aload is present in the field, coupled either inductively or capacitivelyto the antenna. The far-field, on the other hand, is a radiating fieldthat carries energy away from the antenna regardless of the presence ofa load in the field. This energy loss appears to a circuit driving theantenna as a resistive impedance which is known as the radiationresistance. If the frequency of the RF energy used to drive an antennais such that the wavelength of electromagnetic waves propagating thereinis much greater than the length of the antenna, a negligible far-fieldcomponent is produced. In order for a substantial portion of the energydelivered to the antenna to be emitted as far-field radiation, thewavelength of the driving signal should not be very much larger than thelength of the antenna.

A dipole antenna is made of two lengths of metal, usually arranged endto end with the cable from a transmitter/receiver feeding each length ofthe dipole in the middle. An efficiently radiating resonant structure isformed if each length of metal in the dipole is a quarter-wavelengthlong, so that the combined length of the dipole from end to end is ahalf-wavelength. A wire antenna for an implantable medical devicecapable of emitting far-field radiation, however, may require specialimplantation procedures and may also be broken or deformed as a patientmoves resulting in de-tuning. In accordance with the present invention,a dipole antenna for an implantable medical device is formed by separateconductive portions of the device housing or can, referred to herein asa split-can dipole antenna. In one embodiment, the conductive housing issplit into two halves separated by an insulating dielectric material,with each half connected to transmitting/receiving circuitry containedwithin one of the housing portions. Unlike wire antennas, a split-candipole antenna does not require any special implantation procedures andis a rigid structure which is resistant to breakage or deformation.

An antenna most efficiently radiates energy if the length of the antennais an integral number of half-wavelengths of the driving signal. Ahalf-wave dipole antenna, for example, is a center-driven conductorwhich has a length equal to half the wavelength of the driving signal.The natural tuning of a split-can dipole antenna depends, of course onthe device size. For example, a typical lengthwise dimension of animplantable cardiac rhythm management device may be about 6.8 cm, whichcorresponds to a half wavelength of a 2.2 GHz carrier frequency. If eachhalf of a split-can dipole antenna is 3.4 cm, then the antenna is ahalf-wavelength dipole at that carrier frequency. For medical deviceapplications, carrier frequencies between 300 MHz and 1 GHz are mostdesirable. As will be discussed below, an antenna tuning circuit may beused to alter the effective electrical length of an antenna by loadingit with capacitance or inductance. The split-can antenna is especiallyadvantageous in this respect as compared with conventional wire antennasbecause it is physically wide and possesses a greater bandwidth. Anantenna with a greater bandwidth is easier to tune and is usable over agreater range of frequencies once it is tuned. A larger antennabandwidth also allows a higher data rate and minimizes the risk oflosing communications due to frequency drift.

FIG. 1 shows an exemplary implantable medical device 100 with a dipoleantenna suitable for radiating and receiving far-field electromagneticradiation formed by respective halves of the device housing 101 a and101 b. The device housing is metallic and contains therapy circuitry TC1for providing particular functionality to the device such as cardiacrhythm management, physiological monitoring, drug delivery, orneuromuscular stimulation as well as circuitry RFC1 for providing RFcommunications. One or more therapy leads 310 are connected to thetherapy circuitry contained within the housing by means of a header 103with feedthroughs located therein for routing the therapy leads to theappropriate internal components. The two housing portions 101 a and 101b are separated by a layer of insulating material 102. FIG. 2 shows analternate embodiment in which the header is made of dielectric materialand is interposed between the two housing portions 101 a and 101 b, thusalso serving to separate the two legs of the dipole antenna. In eitherembodiment, the two housing portions 101 a and 101 b are hermeticallysealed with a minimum number of feedthroughs between them. A battery B1is used to supply power to the electronic circuitry within the housing.If the battery alone is contained within one of the housing portions,then only two feedthroughs are needed between the two housing portions,one for each battery terminal. Alternatively, the battery and the RFcircuitry can be placed in one housing portion, with the rest of thedevice circuitry contained in the other portion. This shields thesensitive therapy circuitry from the very noisy RF circuitry.

FIG. 3 is a block diagram of an exemplary implantable cardiac rhythmmanagement device utilizing a split-can dipole antenna forradio-frequency telemetry. In the figure, only one therapy lead 310 isshown but it should be understood that a cardiac rhythm managementdevice may use two or more such leads. A microprocessor controller 302controls the operation of the therapy circuitry 320 which includessensing and stimulus generation circuitry that are connected toelectrodes by the therapy leads for control of heart rhythm and RF drivecircuitry 330 for transmitting and receiving a carrier signal at aspecified frequency modulated with telemetry data. The conductors of thetherapy lead 310 connect to the therapy circuitry 320 through a filter321 that serves to isolate the circuitry 320 from any RF signals thatmay be picked up by the lead. The filter 321 may be a low-pass filter ora notch filter such as a choke. The RF drive circuitry 330 includes anRF transmitter and receiver that are connected by a transmit/receiveswitch 333 to the dipole antenna formed by the housing portions 101 aand 101 b. The microprocessor 302 outputs and receives the datacontained in the modulated carrier generated or received by the drivecircuitry 330.

In this embodiment, the RF drive circuitry 330 is connected to thedipole antenna through an antenna tuning circuit which loads the antennawith a variable amount of inductance or capacitance to thereby adjustthe effective electrical length of the antenna and match the antennaimpedance to the impedance of the transmitter/receiver. In this manner,the reactance of the antenna may be tuned out so that the antenna formsa resonant structure at the specified carrier frequency and efficientlytransmits/receives far-field radiation. The tuning circuit in thisembodiment includes a balun transformer 400 and a variable capacitor 402for loading the antenna with an adjustable amount of reactance. Thebalun transformer drives the two housing portions 180 degrees out ofphase and thus also serves to convert between the single-ended signalgenerated or received by the transmitter/receiver circuitry and thedifferential signal generated or received by the antenna. The baluntransformer 400 also acts as a high-pass filter which blocks lowfrequency energy from being passed to the RF circuitry such as may begenerated when the housing is used as an electrode in deliveringelectrostimulation with a monopolar lead.

Although the invention has been described in conjunction with theforegoing specific embodiment, many alternatives, variations, andmodifications will be apparent to those of ordinary skill in the art.Such alternatives, variations, and modifications are intended to fallwithin the scope of the following appended claims.

What is claimed is:
 1. An implantable medical device, comprising:therapy circuitry for providing a functionality to the medical device;RF circuitry for providing RF communications; a battery for supplyingpower to the therapy and RF circuitry; a first housing portion made ofconductive material and containing at least part of either the therapycircuitry, the RF circuitry, or the battery; a second housing portionmade of conductive material and containing at least one of the remainingpart of either the therapy circuitry, the RF circuitry, or the battery;an insulator electrically separating the first and second housingportions; wherein the RF circuitry is connected to the first and secondhousing portions in a manner such that the first and second housingportions form a dipole antenna for transmitting or receiving a modulatedradio-frequency carrier at a specified carrier frequency.
 2. The deviceof claim 1 wherein the first and second housing portions are ofapproximately equal size.
 3. The device of claim 1 further comprising aheader compartment made of insulating dielectric material and whereinthe first and second housing portions are separated by the headercompartment.
 4. The device of claim 1 wherein the therapy circuitry andthe RF circuitry are contained in different ones of the first and secondhousing portions.
 5. The device of claim 3 wherein the battery iscontained in one of either the first or second housing portions with thetherapy and RF circuitry both contained in the other housing portion. 6.The device of claim 3 wherein the battery and RF circuitry are containedin one of either the first or second housing portions with the therapycircuitry contained in the other housing portion.
 7. The device of claim1 wherein the battery is contained in one of either the first or secondhousing portions with the therapy and RF circuitry both contained in theother housing portion.
 8. The device of claim 1 wherein the battery andcircuitry are contained in one of either the first or second housingportions with the therapy circuitry contained in the other housingportion.
 9. The device of claim 1 wherein the dimensions of the firstand second housing portions are such that a significant portion ofradio-frequency energy delivered to the antenna at the specified carrierfrequency is emitted as far-field radiation.
 10. The device of claim 1wherein the electrical length of the antenna is approximately one-halfwavelength or greater of the radio-frequency carrier at the specifiedfrequency.
 11. The device of claim 1 further comprising an antennatuning circuit electrically connected to the antenna and the RFcircuitry for matching the impedance of the antenna to the RF circuitryat a specified carrier frequency by loading the antenna with inductanceor capacitance.
 12. The device of claim 11 wherein the tuning circuitcomprises a balun transformer for converting between a single-endedsignal generated or received by the transmitter/receiver circuitry and adifferential signal generated or received by the antenna.
 13. The deviceof claim 11 further comprising a variable capacitor connected to theantenna for adjusting the resonant frequency of the antenna.
 14. Thedevice of claim 1 wherein the therapy circuitry is rhythm controlcircuitry and further comprising one or more electrodes connected to thetherapy circuitry by therapy leads and adapted for disposition within ornear the heart.
 15. The device of claim 14 further comprising a filterconnected to a therapy lead for blocking radio-frequency signalsgenerated by the RF circuitry from reaching the rhythm controlcircuitry.
 16. The device of claim 15 wherein the filter is a notchfilter.